Eneloop by Sanyo. I think those are the best. You get quicker recharge time and they stay almost fully charged for months if not in use. A bit more expensive, but those are worth every penny.

+1
Before using the eneloops I was using some discounter batteries and they didn't work that long and they were empty about a month after recharging them without using them.
The eneloops just don't lose so much power if you don't use them - after one year they still have about 70% of their charge.
And using them in the flash gun you don't have to change the batteries as often as you have to with most other batteries. And the flash is faster back at full power after having shot one flash.

At the moment I'm using about 30 eneloops and I'm very, very happy with them. Before using them I didn't believe some batteries could make such a difference!

I'd say eneloops too, for casual, infrequent use. They do hold their charge for practically forever. You might still need to take it out of the flash unit when not in use for a long time as I've found some flashes take a little standby power and will slowly drain the batteries.

If you think you might need to use them frequently and heavily, maybe the higher capacity batteries are worth looking at too. These do lose charge much faster over time, so ideally would be used shortly (within days) after charging. Or you could just use more sets of eneloops and change them a little more often.

Syl Arena's book on Speedliting really gets into detail on this subject. He explains why the different types are good for different types of shooters. Alkalines have one advantage, that being availability. Lithium batteries have long life, but are slower between shots. Professionals tend to use Ni-MH rechargeables, which recycle the flashgun faster than alkaline or lithium. Low-discharge Ni-MH have a long storage life once charged, but are slower from shot-to-shot compared to high-discharge Ni-MH. High-discharge will not hold a full charge for long, during storage, but have a faster shot-to-shot ability.

For a shooter who will diligently charge his/her batteries before an event, high-discharge Ni-MH therefore have an advantage for firing many shots repeatedly. This rapid shooting can overheat a flashgun, so multiple flashguns might be needed.

Eneloops are low-discharge Ni-MH. I just opened our first packs of Eneloops,
so have no long-term experience yet. I also just opened our first packs of Powerex, which are high-discharge Ni-MH. I bought both through Amazon. Why both types? Eneloops hold their charge longer, but I can charge-up the
Powerexes right before I expect to need to really use flash heavily.

I have been using Delkin Ni-MH for a while, due to local availability. I think they are high-discharge, due to their high milli-amp rating. It does seem that they will lose much of their charge if sitting inside a flashgun or case for a few weeks, but really work well if freshly charged, which is another
indication they are the high-discharge type. I cannot find much information on Delkins, but they are locally popular, though not so much as Eneloops.

Regarding external packs, not all flashguns have the necessary connection ports. In the current Canon line, I think only the 580EX can use the external battery packs.

I am no expert on flashgun batteries! I did read recommendations by Syl Arena, Joe McNally, and countless others, and listened to local photographers, before recently ordering the Eneloops and Powerexes. I do use my 580EX flashguns frequently, because I work at night. I was going through far too many alkalines, so I had switched to Ni-MH Delkins, but I think Eneloops may be better for most of my needs.

I recently acquired Maha chargers, which treat each battery as an individual, and can be set to recondition each cell with a discharge and then a slow re-charge.

My wife photographs death scenes, mostly at night, and had been using Energizer lithium batteries, but was not too happy with them, due to slow recovery between shots. She will be trying Eneloops now.

Thanks for all the info. Glad that Syl covers this in his book I'm just about to place the order. I placed the order for my 430EX II and then the very same day the Army issues me one and I really don't know a lot of what I'm doing. I'm getting the basics from a few good websites but still have a way to go.

I was using a few Alkalines before but they needed too long recycling the flash and their life was shorter than with eneloops - and eneloops are cheaper in comparison as you can recharge them.
Clear win for the rechargables.

Much of what is in Syl's book has appeared in briefer form as articles on his website, recently re-named Speedliting.com . There is also a link to a video of a recent talk/lesson he gave at a large NY camera dealer, and a link to a series of his lessons that were posted on Canon's European website.

A recent article describes a sunset falconry shoot in Dubai, done with one camera, one lens, and one flash. Good stuff!

I use Ansmann MAXe's, they seem to be similar to the eneloops everyone else has mentioned. They hold their charge for up to a year, which is handy when you carry spares in your bag. Nothing worse that putting a 'fresh' set of rechargables in your flash only to find they are almost flat.